This invention relates to an improvement in off-road vehicle transmission systems of the type wherein gearshift levers are mechanically linked to couplers to select ranges of gear speed ratios, and electro-hydraulic powershift clutches are provided to select one gear speed ratio within a selected range. More specifically, the present invention provides improvements in such transmissions to obtain a more uniform response of the transmission to the operator controls and reduce unexpected gear speed changes.
In the transmission system described in the above-referenced applications, three operator controls are provided for gear selection. A High/Low range shift lever is mechanically linked to a first coupler and selects a first or a second range of gear speeds, each range including 8 gear speeds. A 4/5 range shift lever selects one of two sub-ranges within the range selected by the High/Low shift lever, each sub-range including 4 gear speeds. Finally, two powershift push-buttons are provided for controlling electro-hydraulic clutches to select one of four gear speeds within the range defined by High/Low and 4/5 range select levers. A microprocessor senses the various operator controls and selects a specific powershift gear when the 4/5 shift lever is shifted. That is, powershift gear 4 is selected when the 4/5 shift lever is shifted from its high range position (5) to its low range position (4) and powershift gear 1 is selected when the shift lever is shifted from its low range position to its high range position.
Above-referenced application Ser. No. 07/709,629 discloses and claims a transmission system wherein an automatic selection of the powershift gear is made at start-up. However, when a transmission is provided with an automatic selection of the initial gear at start-up, a reset of the microprocessor while the vehicle is moving may cause a sudden acceleration or deceleration of the vehicle. This might occur, for example, if the engine stalls and the operator restarts the engine while the vehicle is moving, or if the microprocessor is reset by a temporary loss of power. In accordance with one aspect of this invention, a keep-alive memory stores indications of the last selected powershift gear and the positions of the shift levers. These indications are retrieved from memory on re-start and if the vehicle is moving and neither range shift lever has been shifted, the transmission is placed in the same gear it was in before the reset.
Because the shift levers cause automatic selection of powershift gear 1 or 4 depending on whether the shift is to a higher or a lower range, respectively, a problem develops if the operator should depress the clutch pedal, shift from one range to another, change his mind, and shift back to the original range before releasing the clutch. For example, if the transmission is in gear 8 and the 4/5 shift lever is shifted to its low range and back again, gear 5 would be selected rather than the initial gear 8. In accordance with a second aspect of this invention, provision is made for detecting double shifts of a lever during a single clutch pedal depression, and selecting the original gear when the clutch pedal is released thereby avoiding a speed change not expected by the operator.